Runners seem to live by a creed that's stricter than the postman's: "Neither rain, nor snow, nor sniffle, nor fever shall keep me from my training schedule." Indeed, the coming of winter presents many issues for runners who'd prefer to keep at it even when sick. Oftentimes, symptoms aren't severe enough to make you stay in bed, home from work, or off the roads. And while exercise can give you a mental and physical boost when you're feeling run-down, there are other occasions when going for a run may do more harm than good.

This view is supported by research done at Ball State University by Tom Weidner, Ph.D., director of athletic training research. In one study, Weidner took two groups of 30 runners each and inoculated them with the common cold. One group ran 30 to 40 minutes every day for a week. The other group was sedentary. According to Weidner, "the two groups didn't differ in the length or severity of their colds." In another study, he found that running with a cold didn't compromise performance. He concluded that running with a head cold--as long as you don't push beyond accustomed workouts--is beneficial in maintaining fitness and psychological well-being.

But, doctors say, you still walk, or run, a fine line. Take extra caution when training with anything worse than a minor cold because it can escalate into more serious conditions affecting the lower respiratory tract and lungs. Sinus infection, or sinusitis, is an inflammation of the sinus cavity that affects 37 million Americans each year. Symptoms include runny nose, cough, headache, and facial pressure. With a full-blown sinus infection, you rarely feel like running. But if you do, consider the 72-hour rule of Jeffrey Hall Dobken, M.D.: "No running for three days," advises the allergist/immunologist and ultramarathoner in Little Silver, New Jersey. Even without the presence of a fever, says Dr. Dobken, some sinus infections, when stressed by exercise, can lead to pneumonia or, in extreme cases, respiratory failure.

Another option for sinusitis sufferers is pool running. "The water adds moisture to nasal passages," says John J. Jacobsen, M.D., an allergist in Mankato, Minnesota. Pool running is preferable to swimming, says Dr. Jacobsen, because chlorine can be irritating to the nose. If you're still in doubt about whether it's safe to run or not, take your temperature. If it's above 99 degrees, skip your run. "Some people think that they can 'sweat out' a fever by running," says Nieman. "That's wrong. Running won't help your immune system fight the fever."

Nieman saw this firsthand when his running partner once ran a marathon with a 101-degree fever. Soon after, the runner developed severe and persistent symptoms similar to those of chronic fatigue syndrome. "Every day he'd wake up feeling creaky and arthritic," says Nieman. "When he tried to run, he'd stumble and fall." Eventually doctors concluded he had a "postviral syndrome," a latent condition that was exacerbated by the race.

Although this syndrome is rare, it's an example of the risk you take by running while ill. "Running with a fever makes the fever and flu-like symptoms worse," says Nieman, "and it can lead to other complications." During exercise, your heart pumps a large amount of blood from your muscles to your skin, dissipating the heat your body generates. If you have a fever, your temperature will rise even higher, and your heart will be put under greater strain to keep your temperature from soaring. In some cases, this can produce an irregular heartbeat. Also, a virus can cause your muscles to feel sore and achy; exercising when your muscles are already compromised could lead to injury.

Nieman recommends that runners with a fever or the flu hold off until the day after the symptoms disappear--and then go for a short, easy run. Runners should wait one to two weeks before resuming their pre-illness intensity and mileage. Otherwise, you risk a relapse, he says.

How much running can compromise your immune system to the point of making you sick? For average runners, the dividing line seems to be 60 miles a week, according to David Nieman, Ph.D., of the Human Performance Laboratory at Appalachian State University. Nieman conducted the largest study ever done on this question by examining 2,300 runners who competed in the 1987 Los Angeles Marathon. "The odds of getting sick were six times higher than normal after the marathon," says Nieman, "and those who ran 60 miles a week or more doubled their chance of getting sick." The illnesses were of the upper respiratory tract, including sinus infections. Nieman says there's no doubt these findings are still applicable to runners today. He's also used himself as a test case: When Nieman trained up to 90 miles a week, he constantly battled sore throats. When he dropped his weekly mileage below 60, the symptoms stopped.